[ATM] Thin Titanium Spiders & Diffraction

[artbianconi@blast.net]

Ross, the entire telescope (15" f-4.3) is scheduled to be made of 
carbon fiber. The cells, truss tubes, everything. I wanted to be 
able to say that the spider is too but having worked with this 
material for so many years, I questioned my ability to make a 
usable curved spider that is really thin. 

Several times, at the annual NEAF in New York,  I've had 
conversations with the owner of the company that manufactures the 
Port-a-Ball telescopes. He has a curved spider, made from carbon 
Fiber and it is thick. Still, it appeared to be too thin to have 
been made with a core material but since he does not make those 
parts himself, he can't say.

A lot of the arguments I've seen here against the use of CF for 
the spider are technically sound. Arguments notwithstanding, I 
made a test sample out of 3 layers of ordinary 8 ounce Uni-
directional fiberglass using a hand layup and room temperature 
cured. It had little stiffness to it but I anticipated that 
without a core, a flat piece would exhibit that behavior. When I 
curved it into an arc and anchored both ends, it's resistance to 
bending jumped DRAMATICALLY!  I repeated the test using Bi-
Directional glass and the stiffness improved even more! Not sure 
why and I can't test the design as the license for my FEA package 
has long since expired. Unless a client sends me work that 
requires it,  I can't justify the expense of renewing the 
license. 

The final spider, if made from advanced composites, will not be 
made on a flat surface. I used a Formica covered lay-up bench 
because that was available. I was lazy but curious. The mandrel I 
am using for making the secondary cell is the same OD as the ID 
of the secondary and well suited to molding several strips of CF 
into the curve needed for a spider. And, it will still fit into 
my autoclave where 4 atmospheres of pressure will be applied to 
the evacuated vacuum bag. The assumption behind this method is 
that by molding it curved, instead of bending it, will leave a 
part that is stress free and less subject to geometry changes 
from creep. With that much pressure during the cure, I might 
obtain 0.020 thickness (don't know) 

Since the curing temps will be about 150 degrees over room temp, 
creep should not be a factor but since I have the equipment, it 
would be foolish not to take advantage of it.

This entire string started when I uncovered a Google response to 
"Thin Titanium Ribbon" and that prompted me to look for a 
resource for it. Thanks, all of you, for the enormous amount of 
information and for contributing to the success of my project.

I am a mechanical designer, not an astronomer and my knowledge of 
things optical is severely limited. I am surrounded by generous 
and patient talent. Thanks for that too.

Art Bianconi


Ross wrote:

> What is the argument for titanium spider vanes? 
> Stainless steel is nearly twice as stiff (E for steel
> is 30 10^6psi, compared to 17 10^6 psi for Ti) and has
> a higher yield point.  The spider doesn't contribute
> all that much weight to the UTA, so Ti's lower density
> hardly makes any difference (especially since you'll
> have to use twice as much of it to match the stiffness
> of steel vanes) and I can't imagine how Ti's abrasion
> and high temperature resistance could be of any
> possible benefit in a telescope built mainly from
> plywood and soft aluminum tubing.  And it is just SO
> much fun to work.
> 


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